1. Field of the Invention
The present invention relates to a fluoroscopic apparatus and method for radiographing an image by using a radioactive ray such as an X-ray.
2. Related Background Art
In the field of diagnostic imaging using a radiographic apparatus, particularly in a fluoroscopic apparatus, an I. I. (an image intensifier)-television system is used widely. More specifically, in the apparatus, as shown in FIG. 6, a subject P is irradiated with X-rays emitted from an X-ray tube 1 through an X-ray aperture 2, the transmitted X-rays which have passed through the subject P, are detected with a detecting unit 3 and are converted to picture signals, and then the converted picture signals are displayed on a monitor 5 as images through image processing means 4.
X-rays to irradiate the subject P, is set so as to irradiate an only the predetermined area required for radiography through the X-ray aperture 2. A detecting unit 3 converts the X-rays having passed through the subject P to an optical image made of visible lights by an I. I. (an image intensifier) 3a, leads the converted optical image to a television camera 3c through an optical system 3b, and converts the optical image to picture signals by the television camera 3c. 
Image processing means 4 converts input picture signals to digital picture signals, and carries out various computations such as reduction and enlargement for the images, movement for image positions and calculations of picture signals, and image processing. A recording device 6 records and keeps digital picture signals before and after the image processing, and can keep moving images photographed by continuously or intermittently irradiating a subject P with X-rays, and keep still images photographed at arbitrary timings while the inspector observes the moving images.
An area of a subject P which can be photographed with passing X-rays incident to an input plane 3a′ of the I. I. 3a of such a fluoroscopic apparatus, is called a visual field size. The maximum visual field size of the normal I. I. 3a is determined by the diameter of the X-ray aperture, and a settable visual field size is changed stepwise. In radiographing a subject P, the radiographer determines the size of a radiographic area by setting the visual field size.
In addition, by narrowing a visual field size, an enlarged optical image with high resolution can be obtained. A 12-inch wide I. I. 3a can be changed to four sizes, for instance, of 12, 9, 7.5 and 6 inches, and when the visual field size is set to 6 inches, a photograph can be taken in the magnification (resolution) of four times higher than that in 12 inches.
Furthermore, in Japanese Patent Registration No. 2786849, an X-ray diagnostic apparatus is disclosed which can appropriately visualize an X-ray image in accordance with a radiographing portion, and with a fluoroscopic mode or a radiographic mode.
However, the above-described conventional apparatus has the problems described below. In radiography of a subject P, conditions in a fluoroscopy for determining the position of an objective part to be radiographed are desirably a wide area, high sensitivity and a low X-ray dose, while conditions in radiographing a moving image during fluorography or a spot radiography for radiographing still images are desirably high S/N and high resolution. In addition, in either radiography, it is requested that a radiographing area of a subject P can be freely set.
There are many such requests, for instance, when a wide radiographic image is captured in fluoroscopy and then a necessary area for fluoroscopy or spot radiography of an objective part (an area of interest) is going to be specified, in another word, when the quantity of irradiating X-rays for radiography is increased, or a resolution for radiography is increased, in order to obtain clearer images with high S/N, and when the area of interest is pursued while slowly narrowing the photographing area.
In such a case of radiography, the setting of an X-ray irradiation condition and an X-ray aperture and the setting of the visual field size of the I. I. 3a must be independently carried out. But, the above conventional apparatus has a disadvantage in operability.
In addition, a technology is known which synchronizes an irradiated area with X-ray to a visual field size by controlling the opening of the X-ray aperture 2 in accordance with a selected visual field size, when changing the visual field size of the I. I. 3a to a smaller size during fluoroscopy, but according to the technology, the subject is irradiated with unvisualized and useless X-rays while the X-ray aperture 2 is moving.